The present invention is directed to broaching, especially as used for machining finished slots with a single pass, and particularly to a broaching assembly employing novel cutting inserts, and to a novel method of broaching.
Broaching tools are used for machining metal, such as for forming slots in workpieces like turbine wheels. One such broaching tool is disclosed in Clapp et al U.S. Pat. No. 4,243,347 and another is disclosed in Applicant's copending Application Ser. No. 392,260 filed Aug. 10, 1989. The broaching tool comprises a holder and a row of broaching inserts mounted in the holder.
During a broaching operation, each slot is cut progressively by sequentially contacting the workpiece with the broaching inserts or cutters, i.e., initially with inserts which rough-out the slot, an then with inserts which complete the slot. In particular, the workpiece is sequentially contacted with three different types of inserts, i.e., initially with a series of slotters, then with a series of semi-finishers, and finally with a series of finishers, each series of inserts being mounted in a respective holder. The holders are positioned in end-to-end fashion such that the workpiece is contacted successively by a single pass of the slotters, semi-finishers and finishers.
The slotters gradually rough-out a slot to a given depth by sequentially removing sections of the slot. The semi-finishers then enter the roughed-out slot and gradually widen the sides of the slot to establish the size and configuration of the slot to a degree approximating the desired final size and shape. The finishers function to remove the last film of material needed to obtain a slot within the desired narrow manufacturing tolerances.
In a typical broaching assembly approximately 60% of the cutting inserts are slotters, 30% are semi-finishers and 10% are finishers.
It is desirable that the finished slots be smooth and of as precise a configuration as possible. It is also desirable that the cutting inserts exhibit as long a life as possible due to the considerable time and expense involved in sharpening and/or replacing the numerous inserts employed in a typical broaching assembly.
During a broaching operation, numerous factors may adversely affect insert behavior in a manner making those goals difficult to achieve. One of those factors relates to the shock occurring as the insert makes contact with the workpiece which can cause chipping and breaking of the insert to chip and break.
Another factor of concern relates to the reaction of the inserts as they exit the workpiece. It will be appreciated that during a cutting phase the insert is subjected to considerable compressive loading which is suddenly released as the insert leaves the workpiece. The resultant springing-back of the insert, i.e., so-called "snap-back", can result in severe spalling and chipping of the cutting edge.
Yet another fact of concern has arisen more recently in connection with the machining of slots at an angle to the plane of the workpiece, such as a turbine wheel, whereby during the broaching operation the wheel axis is oriented non-parallel relative to the direction of travel of the broaching assembly. During such a broaching operation, one side of the cutting edge of a conventional broaching insert will contact the workpiece ahead of the other side. Such an occurrence is depicted in FIG. 12 wherein one side CE' of a cutting edge CE of a conventional slotter 16' is depicted as making contact with a tilted wheel W ahead of the other side CE". As a result, the insert is subjected to a lateral deflection toward the side CE" which will produce a slight lateral displacement of the slot inlet from its intended position. Moreover, when the slotter exits the workpiece, a lateral deflection thereof in the opposite direction will occur (i.e., toward the side CE'), thereby creating a lateral displacement of the slot outlet from its intended position. Consequently, there will exist a misalignment between the ends of the slot which is intolerable in many applications, necessitating that the slots be formed by techniques other than broaching.
The tendency for the broaching insert to be laterally displaced upon making initial contact with the wheel also applies in the case of semi-finishers and results in a thick chip being cut by one side of the semi-finishers, and a thin chip being cut by the other side. As in the case of a slotter, the semi-finisher will tend to be laterally deflected whereby the misalignment between the ends of the slot will be maintained and possibly even exaggerated. Also, a section of the semi-finished slot located intermediate its ends may be rough and irregular due to vibration of the insert resulting from the lateral deflection.
It would, therefore, be desirable to provide novel broaching inserts which produce slots within narrow tolerances and which resist premature chipping, breaking, and spalling.
It would also be desirable to provide novel broaching inserts which minimize the imposition of vibration, lateral deflection, and shock to the insert.
It would be further desirable to provide novel broaching methods which control the snap-back of broaching inserts exiting a workpiece.